Python Strings and File Input/Output

After completing this topic

You will be able to manipulate strings in various ways using Python and read and write text files.

Strings are sequences of characters

In Python, a string (str) is a sequence of characters arranged in order. Like lists, they can be indexed and sliced.

python

text = "BioPlayground"

print(text[0])      # 'B'  — first character
print(text[-1])     # 'd'  — last character
print(text[3:7])    # 'Play'  — from index 3 to 6
print(len(text))    # 13

Strings are immutable. text[0] = 'b' will cause an error. To modify a string, you must create a new string.

Commonly used string methods

python

msg = "  Hello, World!  "

msg.strip()          # 'Hello, World!'  — removes leading and trailing whitespace
msg.lower()          # '  hello, world!  '
msg.upper()          # '  HELLO, WORLD!  '
msg.replace("World", "Python")  # '  Hello, Python!  '

# Splitting and joining
csv = "apple,banana,grape"
fruits = csv.split(",")    # ['apple', 'banana', 'grape']
"-".join(fruits)           # 'apple-banana-grape'

# Checking for inclusion
"banana" in csv            # True
csv.startswith("apple")     # True
csv.count(",")             # 2

split() and join() are used daily in CSV parsing, log analysis, and data preprocessing.

f-strings — Inserting variables into strings

Introduced in Python 3.6, f-strings are the cleanest way to insert variables directly into strings.

python

name = "Kim Hoon"
score = 95.5

# f-string (recommended)
print(f"{name}'s score: {score} points")
# Kim Hoon's score: 95.5 points

# Expressions are also possible
print(f"Pass or fail: {'Pass' if score >= 60 else 'Fail'}")
# Pass or fail: Pass

# Floating-point formatting
pi = 3.141592
print(f"Pi: {pi:.2f}")  # Pi: 3.14

Older methods ("% s" % name, "{}" .format(name)) also work, but f-strings are superior in both readability and performance.

Reading files

python

# Read the entire file
with open("data.txt", "r", encoding="utf-8") as f:
    content = f.read()
    print(content)

# Read line by line (better for large files)
with open("data.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())  # Remove newline character

The with statement opens a file and automatically closes it when the block is finished. You do not need to call f.close() explicitly. If you do not use with, the file may not be closed, which can lead to memory leaks. Always use with.

Writing to files

python

# Create a new file (overwrites existing content)
with open("output.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")

# Append to an existing file
with open("output.txt", "a", encoding="utf-8") as f:
    f.write("Appended line\n")

Mode	Meaning	Behavior if file does not exist
`"r"`	Read-only	Error
`"w"`	Write (overwrite)	Create new file
`"a"`	Append	Create new file
`"x"`	Create (error if file exists)	Create new file

Practical pattern — Parsing CSV directly

python

# Parse a CSV file directly (without libraries)
with open("scores.csv", "r", encoding="utf-8") as f:
    header = f.readline().strip().split(",")
    print(header)  # ['Name', 'Korean', 'English', 'Math']
    
    for line in f:
        fields = line.strip().split(",")
        name = fields[0]
        total = sum(int(x) for x in fields[1:])
        print(f"{name}: Total score {total}")

Of course, in real-world applications, you would use the csv module or pandas, but understanding the principles of file input/output allows you to handle any type of data.

Key takeaways

Strings and files are the input/output channels of programming. All processes of reading external data, processing results, and outputting them in a human-readable format begin here. split, join, f-string, with open — mastering these four will enable you to handle most text processing tasks.

BioPlayground

Python Strings and File I/O

Python Strings and File Input/Output

After completing this topic

Strings are sequences of characters

Commonly used string methods

f-strings — Inserting variables into strings

Reading files

Writing to files

Practical pattern — Parsing CSV directly

Key takeaways

💬 Questions & Comments